Study On The Critical Parameters Of Nb₃Sn And Nb₃Al Under Mechanical Deformation Based On The First-principles Calculations

Since the excellent superconductivity critical capabilities, the typical A15-family superconductors of the A3 B compounds, Nb3 Sn and Nb3 Al are currently used in a variety of large-scale scientific projects employing high-field superconducting magnets, including the International Thermonuclear Experimental Reactor(ITER) and the CERN LHC Luminosity Upgrade. However, in the high-field magnets made of superconductors, the mechanical loads caused by different thermal contractions during cool down and the Lorentz forces from electromagnetic field can be very large, which has a significant influence on the superconducting critical parameters.In this study, first-principles calculations for the low temperature superconductors based on planewave pseudo-potential density functional theory within the generalized gradient approximation are implemented, and the elastic moduli and the density of states at the Fermi level of Nb3 Sn and Nb3 Al are evaluated, then the superconducting critical parameters under mechanical loads are presented from the McMillan equation and the unified scaling law. Firstly, the elastic moduli and the Debye temperatures of Nb3 Sn and Nb3 Al are calculated, and then the critical temperatures under hydrostatic pressure are obtained by the McMillan equation. The results show that the elastic moduli of the low temperature superconductors are enhanced by the applied hydrostatic pressure, while the critical temperatures usually are decreased with the pressure. Additionally, the decrease of critical-temperature for Nb3 Sn is more sensitive to the hydrostatic pressure than the one for Nb3 Al. The prediction results show good agreement with the experimental results in the literatures qualitatively, that because of the valence electrons of Nb3 Sn are always more than Nb3Al’s. In addition, we calculated the superconducting properties of under tension, shear and torsion. The all critical parameters decrease with the shear strain increasing. A systematic comparison of our calculated result, the critical current descend faster in the same strain variation range, the torsion deformation have greater influence of critical parameters than shear deformation. These numerical results are quite consistent with the experimental results in the literatures qualitatively.